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Filter Ratings

I need to replace normal 2" pleated filters,Merv 7 or 8, with Merv 14 pleated. The cost between Merv 13 and 14 is substantial, any one know what difference there is between the 13 & 14? Also, in one of the AHU's, I have 12" Pleated Box Filters" and 2" pre-filters, does anyone know if I can add the 12" Merv rating to the normal 2" pleated {7 or 8 merv} and achieve my 14 Merv? Thanks.

If no one is complaining about allergies and dust I would stick with the MERV 13. Make sure your fan can handle the extra resistance from the upgraded filter or you will have people complaining about the space not getting cold enough. From my understanding is the higher the MERV value the smaller the particles that can pass through it... so having a second filter might help trap that 2% that pass through the first one, but both won't stop the smaller particles so you won't get higher MERV ratings persay. Your unit will have to work harder with two filters to push the air to your spaces. See chart below for what size particle a Merv 13 and Merv 14 can stop.